Rear Post mount adapter query

I have a Ragley Big Wig that is post mount on the rear - i want to run a 180mm disc rather than 160mm.
Most 160mm to 180mm adapters seems to be front, and struggling to find anything rear specific.
I have heard that if you're going PM to PM on the back you need an adaptor one size bigger so if going 160mm to 180mm i actually need a 160mm to 203mm PM adapter.
Anyone know if there's any truth in that?

the mount sizes are 20mm increments (generally but there are exceptions)
If the frame is PM and runs 160mm with no extra hardware then to go 20mm up you will (probably) need the +20mm 160-180. If you already have something there then you will need a +40 (160-200).

Nothing looks comparable on the websites, depending where you are the quickest and simplest solution is to ride the bike into a shop and go got an adaptor for this mate

Thanks - in theory its simple, but the fact that a 160-180 doesn't fit and with what you've both said about makes me think it's luck of the draw and depends on the brake and frame combo?
Unfortunately its not the kind of thing my local shop stocks so its a bit trial and error at the moment.
cheers

IS Mounts were different front and rear, so bolting the same IStoPost mount on the front resulted in a 20mm bigger rotor compared to the rear.

Post mount frames/forks 'should' be the same, as a post mount 160 is a post mount 160 irrespective of if its front or rear. Of course frame/fork designers complicated this by then designing dedicated 140mm, 180 and 203mm post mounts.

So..
To fit a bigger rotor you need to identify the frame mount and buy the appropriate +xx adaptor (no front/rear specific versions exist?), of course some frame designers will not consider you might want to use adaptors or only consider one brand, so you might find some wont fit very well or at all.

Had a quick look at the frame on google and cant see why a shimano adaptor wouldnt fit (make sure you use the right bolts mind!) but that hope one might interfere, apart from the fact its also 183mm not 180mm.